Changes in the supply or demand of active power from a power grid can result in an increase or decrease of the grid frequency. For example, an increase in demand will result in a drop in the grid frequency. An allowable range within which the grid frequency can depart from the nominal grid frequency or “dead band” is specified in national or local grid requirements that are imposed on power producers feeding into the grid. The power producers must ensure that their power plants can respond to a frequency event, i.e. a situation in which the grid frequency moves outside or indicates a tendency to move outside the predefined dead band, by initiating a frequency response in order to contribute to moving the grid frequency back within the permitted bounds. Since a power plant's frequency response is usually a requirement of the relevant grid code, it can also be referred to as the “grid code frequency response”. Here and in the following, the terms “grid code frequency response” and “frequency response” is to be understood as a measure of the ability of a power plant to respond to a change in the frequency of the grid into which it feeds.
Therefore, a grid operator or an operator of a power production facility can wish to perform a frequency response test to demonstrate compliance with the local grid interconnection requirements for frequency response or inertial response. The frequency response of a power plant (also referred to as a “power facility”) will describe its ability to respond to a change in grid frequency, for example its ability to release additional power into the grid if the frequency has dropped. In the known approaches, to carry out a frequency response test, a power plant is operated in a “test” mode, during which an actual grid frequency is replaced by a test sequence comprising simulated digital frequency samples or an analogue simulated frequency signal. The power plant then operates using these grid frequency values, and the output of the power plant should reflect its behavior in a real world situation. Usually, a “test switch” is used to place the facility in the “test mode”, and once the test is completed, the test switch places the facility back into the normal mode of operation once more. For a power plant such as a gas turbine power station, the plant controller directly controls the generator, and the frequency response is essentially provided by that generator. For installations involving multiple plant controllers, for example in the case of a large wind farm, the plant controllers must be tested simultaneously. To this end, parallel injection tools with identical injection profiles or time sequences must be set up for the separate plant controllers. The test sequences are injected simultaneously to the plant controllers, and triggered to start at the same time, in order to obtain a realistic assessment of the collective frequency response.
Newer types of power networks can comprise power generation facilities as well as power storage facilities. The facilities can be distributed over several locations, for example a number of geographically separate offshore wind parks and a number of power storage plants located onshore for storing “surplus” energy. These facilities can feed separately into the same utility grid at separate points of connection, or a power plant facility might comprise one or more power production units as well as power storage units, so that such a “distributed” power network can be regarded as a “virtual power plant”, with a virtual grid interconnection point representing the separate connection points of the individual power plants, and a virtual combined grid output representing the net power output of the different facilities.
The currently available test systems cannot be applied to such power networks. Existing solutions involve the injection of a test sequence into a single plant controller, or, in the case of a power plant such as a wind farm, into the controller of a single sub-unit such as a wind turbine. The current tools therefore cannot be used in a situation requiring frequency response at a power generation unit level as well as frequency response from a power plant controller, since the frequency response for the sub-unit must be extrapolated to estimate the overall frequency response of the entire power plant, which can therefore differ significantly from its actual frequency response. Furthermore, testing a power storage unit and a power production unit separately to obtain their frequency responses does not give any indication of the combined frequency response of the power storage unit together with the power production unit. Another disadvantage of the known test systems is that they require considerable effort in setting up if they are to be used to determine the frequency response of more than one plant controller. This means that existing test systems cannot simulate or predict the collective or aggregate response at the point of common connection for the type of virtual power plant described above.